For example, in a semiconductor device manufacturing process, a photolithography method including a sequence of processes such as a resist coating process of coating a resist solution onto a semiconductor wafer (hereinafter, referred to as a “wafer”) used as a substrate to form a resist film, an exposure process of exposing the resist film into a predetermined pattern, a developing process of developing the exposed resist film and the like, is performed to form a predetermined resist pattern on the wafer. Subsequently, an etching process of etching a target film of the wafer using the resist pattern as a mask, a removal process of removing the resist film and the like are sequentially performed. Thus, the predetermined pattern is formed on the target film.
In recent years, there is a demand for miniaturizing a pattern of the aforementioned target film to realize further high integration of the semiconductor device. To do this, a miniaturation of the resist pattern is in progress. As an example, a wavelength of light used in the exposure process of the photolithography method has been shortened. However, there are technical limitations and cost restrictions in shortening a wavelength of light emitted from an exposure light source. This makes it difficult to form a fine resist pattern on the order of, e.g., several nanometers, only by shortening the wavelength of light.
To address this, as a pattern forming method substitutable for the photolithography method, there is proposed a method of processing a wafer using a block copolymer constituted by combining two types of polymers such as polymethyl methacrylate (PMMA) and polystyrene (PS). In such a method, a pattern region having high affinity with respect to one of the polymers is first formed on the water. Subsequently, for example, a resist pattern is formed on the pattern region. Thereafter, the block copolymer is coated onto the wafer with the resist pattern formed thereon, and subsequently, the block copolymer is phase-separated. Thus, one of the phase-separated polymers is arranged in a region having high affinity with respect to the respective phase-separated polymer and the other is positioned adjacent to the arranged polymer.
In addition, when one of the polymers (in this case, polymethyl methacrylate) is selectively removed by etching using, for example, an oxygen plasma or the like, a fine pattern is formed on the wafer by polystyrene used as the other polymer. Subsequently, a target film is etched using the pattern of polystyrene as a mask so that a predetermined pattern is formed on the target film.
However, in the etching process using the oxygen plasma as described above, since a selectivity of polymethyl methacrylate to polystyrene falls within a range from about 3:1 to 7:1, when the polymethyl methacrylate is removed, a film thickness of the polystyrene may be also decreased. This fails to secure a required film thickness of the polystyrene when the polystyrene is used as an etching mask in a subsequent process.
The present inventors extensively studied how to increase selectivity from one of the polymers to the other when selectively removing one of the polymers from a phase-separated block copolymer. As a result, the present inventors have found that, when the phase-separated block copolymer is heated in a low oxygen atmosphere, a first polymer containing an oxygen atom is substantially decomposed, volatilized and removed, while a second polymer containing no oxygen atom is substantially not changed in a film thickness.